The present invention relates to the field of reducing sand production during borehole drilling, perforating and hydrocarbon production. In particular, the invention relates to a method of reducing sand production from perforated sandstones with bedding layers.
In the production of hydrocarbons from hydrocarbon-bearing unconsolidated formations, a well is provided which extends from the surface of the earth into the unconsolidated, or poorly consolidated formation. The well may be completed by employing conventional completion practices, such as running and cementing casing in the well and forming perforations through the casing and cement sheath surrounding the casing, thereby forming an open production interval which communicates with the formation.
The production of hydrocarbons from unconsolidated or poorly consolidated formations may result in the production of sand along with the hydrocarbons. Produced sand is undesirable for many reasons. It is abrasive to components within the well, such as tubing, pumps and valves, and must be removed from the produced fluid at the surface. It may partially or completely clog the well, thereby making necessary an expensive workover. In addition, the sand flowing from the formation may leave therein a cavity that may result in collapsing of the casing.
It is known and described for example in the U.S. Pat. Nos. 6,003,599, 5,443,119, or 5,360,066 to orient perforations with respect to the azimuthal direction of the maximum in-situ horizontal compressive stress. This direction within a hydrocarbon-bearing reservoir having non-uniform horizontal tectonic stresses surrounding a well is determined.
Oriented perforations are then formed in the reservoirs surrounding the well. These perforations are oriented in the azimuthal direction of the determined maximum in-situ horizontal compressive stress. Thereafter hydrocarbon production is initiated from the reservoir into the well through the perforations, whereby the potential for production of sand along with hydrocarbons produced from the reservoir is minimized due to the orientation of the perforations within the reservoir in the direction of maximum in-situ horizontal compressive stress. If the well is cased, the perforations extend through such casing and into the reservoir. In some of the above cited references, the perforation tools is oriented and orientated perforations are shot to increase the effectiveness with less regard to sanding problems but more in view of a later fracturing of the formation.
Furthermore, it is known and described for example in the U.S. Pat. No. 5,040,619 to incorporate into the design of a perforation gun a swivel connected with a cable head assembly and a navigation system for determining the instantaneous angle of the tool with respect to a vertical reference. The angle of firing of the shaped charges is adjusted at the time of installation with respect to the horizon and that in turn is correlated to the formation of interest in the well borehole which is then perforated with perforations which are parallel to the formation bedding plane.
In the ""619 and other patents, perforation are oriented in direction of bedding planes within the formation. The purpose of that particular orientation is to ensure maximum permeability of the formation around the circumference of the perforation.
In view of the known art, it is seen as an object of this invention to improve the selection of an optimal orientation of a perforation with respect to the surrounding formation.
According to the invention perforations are generated in subterranean formations by a method that comprises the steps of determining the orientation of bedding planes of said formation; defining an orientation of said perforation relative to said bedding planes; determining a cross-section of a hole generated by said perforation in said bedding plane; calculating a stress concentration along the circumference of said cross-section; and repeating these steps until said stress concentration along said cross-section is homogenized.
Hence, the invention provides an optimization process according to which perforations are oriented with respect to the orientation of bedding planes. The optimization process is mainly based on homogenizing the stress concentration or tangential stress at the perimeter of the cross-section of the perforation with the bedding plane. The expression xe2x80x9chomogenizexe2x80x9d is understood as minimizing the difference between the largest and the smallest stress concentration along the circumference of the cross-section. It is effectively attempting the level the stress along the circumference so as to avoid peaks of stress.
However, according to a further aspect of the invention the optimization includes in addition to the mutual orientation of bedding planes and perforation further parameters. Such parameters are the stress distribution in the formation, i.e. any inhomogeneity of stress in the rock. Such deviation from what is usually referred to as hydrostatic will affect the optimal orientation of the perforation under the stress balancing criterion stated above.
Another aspect of the invention includes the use of geometrical consideration in the optimization process. The geometrical aspect includes the shape of the perforation within the bedding plane. It was found that the stability of a perforation depends inversely on its radius. Given that in many cases the perforation will generate a hole with an elliptical cross-section in the bedding plane (if it is not shot exactly perpendicular to the bedding plane), the effective radius of curvature of the hole changes from point to point. As an ellipse is highly symmetric, it might suffice to calculate the effective radius at only a small number points.
Ideally and in addition to the parameter mentioned above, the optimization process includes a criterion that relates to permeability. Hence, the optimization process, which is predominantly a stability-focussed process, may have permeability considerations imposed on it as additional constraints. As it is known that permeability is higher in direction of the bedding planes, whereas stability in an idealized case tends to be higher for a perforation perpendicular to the bedding plane, it can be easily seen that any pre-determined constraints on permeability (and hence productivity) can have a significant impact on the final optimized orientation of the perforations.
These and other features of the invention, preferred embodiments and variants thereof, possible applications and advantages will become appreciated and understood by those skilled in the art from the detailed description and drawings following below.